This invention relates generally to radiotelephones and, in particular, to radiotelephones or mobile stations such as those capable of operation with a code division, multiple access (CDMA) cellular network.
Advances in the field of telecommunications have resulted in a variety of types of telecommunications systems being available for use by the general public. Among these telecommunications systems, cellular telephone systems are presently one of the most rapidly developing in terms of technologies and services offered. Cellular systems are currently in widespread use worldwide, with continued growth in sales and subscribers predicted for the future.
Several types of technologies have become dominant in the cellular industry. In the United States most cellular systems currently operating use analog signal transmission techniques, as specified by the Telecommunications Industry Association/Electronic Industry Association(TIA/EIA) AMPS standard, or a combination of analog and time division multiple access (TDMA) signal transmission techniques, as specified by the TIA/EIA IS-54 and IS-136 standards. In Europe, cellular systems may operate according to one of several analog system standards, depending on the country, or according to the digital Global Services for Mobile (GSM) TDMA standard that has been specified for Europe. In other parts of the world most cellular systems operate according to one of the standards used in the United States or Europe, except for in Japan where the TDMA personal digital communication (PDC) standard has been developed and is in use. However, in spite of the present dominance of analog and TDMA technologies, the cellular industry is dynamic and new technologies are constantly being developed as alternatives to these currently dominant technologies. One alternative digital signal transmission technique that has recently been the focus of interest for cellular systems is known as code division multiple access (CDMA). In a CDMA system multiple users, each using a channel identified by a uniquely assigned digital code, communicate with the system while sharing the same wideband frequency spectrum.
CDMA provides several advantages over conventional analog or TDMA systems. Frequency spectrum allocation planning for mobile stations and the base stations of cells within a CDMA system is not necessary, as in analog and TDMA systems, because all CDMA base stations share the entire downlink frequency spectrum, and all mobiles share the entire uplink frequency spectrum. The fact that the wideband frequency spectrum is shared by all uplink or downlink users in CDMA also increases capacity since the number of users that can be multiplexed simultaneously is limited by the number of digital codes available to identify the unique communications channels of the system, not by the number of radio frequency channels available. Additionally, since the energy of the transmitted signals are spread over the wide band uplink or downlink frequency band, selective frequency fading does not affect the whole CDMA signal. Path diversity is also provided in a CDMA system. If multiple propagation paths exist, they can be separated as long as the differences in paths delays does not exceed 1/BW, where BW equals the bandwidth of the transmission link. An example of a widely accepted cellular system CDMA standard is the TIA/EIA IS-95-A system standard.
Because data transmission applications other than conventional voice traffic transmission are becoming increasingly important in the cellular system area, a system operator who operates a CDMA system may desire to provide services other than phone voice service. Examples of these other services include portable computer cellular modem service or video service. Often, these other services may require that data be transmitted at a rate much faster than that required for voice transmission.
In the case where it is desired to provide a range of different services in a CDMA cellular system, a method and apparatus for varying the data transmission rate in the system, where the data rate could vary within a range required for all system services, must be provided. It would be desirable that this apparatus provide both slower speed data transmission for efficient and reliable speech service and high speed data transmission for other applications. For example, the IS-95-A system is limited to a maximum data rate of 9600 bits per second (9.6 kbps). However, it may be desirable to provide services in the IS-95-A system that require data transmission at rates greater than 9.6 kbps. Additionally, because of the large costs involved in developing new systems, it would also be desirable if the data transmission apparatus could be operated within an existing system on a noninterfering basis with slower speed transmitters and receivers, and could be implemented with minimum modifications to the air interface of the existing system.
One technique for providing a higher data rate is to use multiple, parallel data channels that are simultaneously transmitted between the mobile station and the base station. In this case the parallel data channels are separated by unique spreading codes. A high speed data user is assigned one fundamental code channel, and one or more supplemental code channels. The fundamental code channel is assigned for the duration of the connection time, and is used for data traffic and signalling, while the one or more supplemental code channels are assigned for all or a portion of the connection time, and are dedicated for high speed data (HSD) traffic.
However, one significant problem with high speed data transmission in the CDMA reverse link (mobile station to base station), when using such a multiple parallel traffic (code) channel configuration, relates to the mobile station power amplifier (PA) efficiency. This is due to the fact that the addition of the sub-channel modulated waveforms results in a higher peak to average ratio of the transmitted signal, and thus additional backoff in the power amplifier in order to maintain the required linearity. Consequently the mobile station is constrained to deliver lower output power, compared to a single channel signal of the same data rate. When the mobile station reaches its peak transmitter power the connection quality can no longer be maintained, despite the base station commanding a further increase in transmitted power. This can lead to an excessive number of high error frames and a possible drop of the connection.
It is thus a first object of this invention to provide an improved method for operating a mobile station at an increased effective data rate, without increasing the required linearity of the mobile station""s output power amplifier.
It is another object of this invention to provide a mobile station that operates with a fundamental and at least one supplemental code (data) channel, and to enable the supplemental code channel to be selectively placed in a discontinuous transmission (DTX) state (a DTX low state) based on the occurrence of at least one criterion being fulfilled.
It is a further object of this invention to provide a mobile station that operates with the fundamental and the at least one supplemental code channel, and to enable the supplemental code channel to be selectively placed in the DTX low state if a commanded transmitter power increase would cause the mobile station to exceed its output power limit.
The foregoing and other problems are overcome and the objects of the invention are realized by methods and apparatus in accordance with embodiments of this invention.
To overcome the foregoing and other problems, the mobile station is operated so as to autonomously reduce its transmission data rate by lowering the number of parallel code channels in use, and/or by reducing the data transmission rate through a code channel. By so doing the mobile station is enabled to improve the link budget and increase the backoff in the power amplifier, while at the same time delivering more output power if required by the base station, thereby maintaining the connection quality at the expense of the user transmission data rate.
An important advantage that accrues from the use of this invention is an improvement in the reverse link coverage for power limited, high speed data terminals, at least to the extent that the traffic channel connection can be maintained at a lower data rate.
This invention applies to a mobile station that is able to determine a required data rate based on data buffer usage. That is, if the data buffer indicates a requirement for a high data rate transmission, the mobile station sends a request to the base station to be assigned multiple code channels. If the multiple code channels are granted, the mobile station uses the multiple channels for transmitting data to the base station until either the data buffer becomes empty, or a base station time-out occurs, or the mobile station is signalled by the base station to reduce its data transmission rate, whichever occurs first.
In accordance with an aspect of this invention, the mobile station is enabled to autonomously control the data rate during an assigned period for high speed transmission, so as to lower the data rate when needed for the purpose of improving coverage and/or to avoid operating in a power limited condition, thereby degrading link quality.
Aspects of this invention provide a technique to achieve a mobile station initiated discontinuous transmission on one or more of the sub-channels assigned by the base station in the multiple-channel, high speed data reverse link configuration, for the purpose of maintaining connection quality and improving coverage when operating in a power limited environment.
This invention thus teaches a mobile station that is operated in accordance with the steps of: (a) establishing a wireless data communication from a transmitter of the mobile station to a receiver of the base station at a predetermined data rate simultaneously through a fundamental data channel and at least one supplemental data channel; (b) receiving a command from the base station to increase a transmission power of the mobile station transmitter; (c) determining in the mobile station if the increased transmission power will exceed a transmission power threshold value; if so, (d) reducing the data rate by disabling data transmission through at least one supplemental data channel; and (e) increasing the transmission power.
The mobile station is further operated in accordance with the steps of: (f) receiving a command from the base station to decrease the transmission power of the mobile station; (g) determining in the mobile station if the decreased transmission power, assuming that a disabled supplemental data channel were once more enabled, will be less than the transmission power threshold value; if so, (h) increasing the data rate by enabling data transmission through at least one previously disabled supplemental data channel; and (i) decreasing the transmission power.
In a preferred embodiment of this invention the step of increasing the data rate includes a step of transmitting a resume message or preamble from the mobile station to the base station on a previously disabled supplemental code channel. The preamble can be used to assist in synchronizing (e.g., chip synchronizing) the base station to the channel prior to the resumption of data transmission on that channel.